Neurogenic placodes are thickenings of vertebrate embryonic ectoderm which form the membranous labyrinth of the ear, including the cochlear duct, and which, together with cells derived from the neural crest, form all cranial sensory ganglia. While recent experimental analyses of placode formation in the chick embryo have documented the derivates of placodes, there is little information on the tissue interactions which are necessary for the embryonic determination of these essential components of the head. In the experiments proposed the trigeminal, otic or post-otic epibranchial (nodose, petrosal) placodes will be excised and heterotopically transplanted in the place either of one another or of non-placodal ectoderm, and vice versa. By varying donor and host stages the tissue interactions necessary for initial placode thickening and for neuroblast formation and migration can be analyzed. In all cases the donor tissues will be labeled so they can be distinguished from host cells. Neuronal specification of neurons derived from various ectodermal placodes grafted in the place of the trigeminal placode will be analyzed by examining the somatotopic organization and peripheral projections of each ganglion. This is done by applying horseradish peroxidase (HRP) to discrete peripheral areas and later mapping the locations and cytological features of peroxidase-filled ganglionic neurons. By comparing these results with data on normal trigeminal ganglion organization, which has already been described, it will be possible to define when, and as a result of which tissue interactions, cranial sensory neuron-producing anlagen become functionally and regionally specified.